Address PR #555 review:
- chain: derive prev_hash from file tail under cross-process flock, drop the
global .chain sidecar so per-day files stay independently verifiable and
concurrent dws processes cannot fork the chain
- forward: track async forwards with WaitGroup and add bounded Close(ctx) so
in-flight deliveries are not dropped on process exit
- actor: resolve Actor from the active runtime profile (profile-keyed cache)
- observability: BuildSink returns init errors; write/forward failures reported
to file log and to stderr when DWS_AUDIT_DEBUG is set
- cli: reject `audit tail --lines` < 1; check CSV writer/flush errors
- wire CloseAuditSink into PersistentPostRunE
- add regression tests for cross-date/cross-process chain, forwarder
wait/timeout, init-failure, tail validation, CSV export
修雨